Whenever two bodies with arbitrary properties are brought into sliding contact, control of the size and mechanical properties of the contact is normally achieved by controlling the force or interference between the bodies. These two techniques become difficult or impossible to use as the size of the contact is reduced. For instance, if a contact of very small dimensions is required, the physical interference between the two becomes so small as to be nearly impossible to measure, particularly as the two bodies move relative to one another. Because the forces involved in establishing such small contacts are themselves extremely small, they are also difficult to measure and control. This is particularly true because as the two bodies move, the physical interference will change as just described. Thus, it is difficult to control very small sliding contact as those that would be used in nano-scale machining, nanoprofilometry, atomic force microscopy images and very low load friction measurements.
U.S. Pat. No. 4,848,141 to Oliver et al. describes a method for continuously measuring the stiffness and area of contact between two bodies, in which elastic stiffness of a junction is measured by introducing a relatively small oscillatory mechanical force at a known frequency to the junction and measuring the subsequent displacement response using AC signal-handling techniques. The apparatus provides a continuous measurement proportional to the stiffness and the area of contact between the bodies.
The aforementioned U.S. Pat. describes an adequate technique for characterizing a point depression as it is produced in a surface using contact stiffness, but does not describe characterizing a scratch. Scratch characteristics are important parameters in the field of nano-scale machining. Prior art techniques using force and size measurements are deficient. These are typically inadequate because the magnitude of the forces and distances involved are exceedingly small, to the point of being unmeasurable and thus unsuitable for control purposes. Thus, a continuing need exists for improved apparatuses and methods for the continuous measurement of scratch characteristics during sliding contact between two bodies.